Motor rotation rate detecting circuit and motor driving device

ABSTRACT

In a motor rotation rate detecting circuit incorporating a conventional differentiating circuit, raising the upper limit of the differentiating operation frequency of the differentiating circuit to cope with an increased range of the motor rotation rate causes the differentiating circuit to amplify high-frequency noise when the motor is rotating at a low rate. To solve this problem, a motor rotation rate detecting circuit embodying the invention incorporates a differentiating circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to the rotation position of the motor, and the frequency response of the differentiating operation of the differentiating circuit is switchable according to the voltage of the rotation rate detection signal.

This is a continuation of application Ser. No. 10/610,753 filed Jul. 2,2003 now U.S. Pat No. 6,841,959. The disclosure of the priorapplication(s) is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor rotation rate detecting circuitthat produces a rotation rate detection signal by differentiating aplurality of alternating signals having different phases that are outputaccording to the rotation position of a motor. The present inventionalso relates to a motor driving device incorporating such a motorrotation rate detecting circuit.

2. Description of the Prior Art

Japanese Patent Applications Laid-Open Nos. H9-127140 and S57-76456disclose conventionally known examples of motor rotation rate detectingcircuits that produce a rotation rate detection signal bydifferentiating two-phase sinusoidal signals output from an encodercoupled to a motor.

The motor rotation rate detecting circuit disclosed in Japanese PatentApplication Laid-Open No. H9-127140 includes an inverting circuit thatinverts the polarities of two-phase sinusoidal signals output from anencoder coupled to a motor, a switching circuit that selects one amongthe four sinusoidal signals, namely the two-phase sinusoidal signalsoutput from the encoder coupled to the motor and their respectiveinverted signals, a differentiating circuit that differentiates thesignal output from the switching circuit, a logic selecting circuit thatcontrols the timing of the switching performed by the switching circuit,and a differentiation error eliminating circuit that eliminates adifferentiation error component from the differentiated signal outputfrom the differentiating circuit to output a rotation rate detectionsignal.

Differentiating circuits of various configurations can be used in amotor rotation rate detecting circuit. However, a signal having afrequency higher than that corresponding to the maximum motor rotationrate is a high-frequency noise signal, and therefore it is customary touse a differentiating circuit that does not perform differentiatingoperation on a signal having a frequency higher than that correspondingto the maximum motor rotation rate.

FIG. 12 shows an example of the configuration of a differentiatingcircuit that does not perform differentiating operation on a signalhaving a frequency higher than that corresponding to the maximum motorrotation rate. The differentiating circuit shown in FIG. 12 is composedof a capacitor C3, resistors R9 and R10, and an operational amplifierOP3. One end of the capacitor C3 serves as the input end of thedifferentiating circuit. The other end of the capacitor C3 is connectedthrough the resistor R9 to the inverting input terminal of theoperational amplifier OP3. The non-inverting input terminal of theoperational amplifier OP3 is grounded. The operational amplifier OP3receives negative feedback through the resistor R10. The node at whichthe output terminal of the operational amplifier OP3 and the resistorR10 are connected together serves as the output end of thedifferentiating circuit. Here, let the capacitance of the capacitor C3be C₃ [F] and the resistance of the resistor R10 be R₁₀ [Ω]. Then, thedifferentiating operation frequency of the differentiating circuit shownin FIG. 12 is lower than 1/ (2×π×C₃×R₁₀) [Hz]. The differentiatingoperation frequency denotes the frequency at which the differentiatingcircuit offers differentiating operation. Accordingly, the capacitanceC₃ and the resistance R₁₀ are so determined that the maximum value ofthe differentiating operation frequency is equal to the frequencycorresponding to the maximum motor rotation rate.

The motor rotation rate detecting circuit described above is used todetect the rotation rate of various types of motor. For example, it isused to detect the rotation rate of a spindle motor used in an opticaldisk apparatus.

In an optical disk apparatus, as the disk rotation rate becomesincreasingly high, the range of the frequency of the output signal ofthe encoder coupled to the spindle motor becomes increasingly wide. Asthe range of the frequency of the output signal of the encoder becomeswider, the maximum value of the differentiating operation frequency ofthe differentiating circuit shown in FIG. 12 needs to be made higher.

However, raising the maximum value of the differentiating operationfrequency of the differentiating circuit causes the differentiatingcircuit to perform differentiating operation on signals in ahigh-frequency range even when such signals are not needed because themotor is rotating at a low rate. As a result, when the motor is rotatingat a low rate, quite inconveniently, the differentiating circuitamplifies high-frequency noise.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a motor rotation ratedetecting circuit that is capable of wide-range detection withsatisfactory resistance to high-frequency noise, and to provide a motordriving device incorporating such a motor rotation rate detectingcircuit.

To achieve the above object, according to the present invention, a motorrotation rate detecting circuit is provided with a differentiatingcircuit that produces a rotation rate detection signal bydifferentiating a plurality of alternating signals having differentphases that are output according to the rotation position of a motor,and the frequency response of the differentiating operation of thedifferentiating circuit is switchable according to the voltage of therotation rate detection signal.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 is a diagram showing an example of the configuration of a motorrotation rate detecting circuit embodying the invention;

FIG. 2 is a diagram showing an example of the configuration of theinverting circuit provided in the motor rotation rate detecting circuitof FIG. 1;

FIG. 3 is a diagram showing an example of the configuration of theswitching circuit and the logic selecting circuit provided in the motorrotation rate detecting circuit of FIG. 1;

FIG. 4 is a diagram showing the waveforms of the signals observed atrelevant points in the motor rotation rate detecting circuit of FIG. 1;

FIG. 5 is a diagram showing an example of the configuration of theselecting circuit provided in the motor rotation rate detecting circuitof FIG. 1;

FIG. 6 is a diagram showing an example of the configuration of thedifferentiating circuit provided in the motor rotation rate detectingcircuit of FIG. 1;

FIG. 7 is a diagram showing another example of the configuration of thedifferentiating circuit provided in the motor rotation rate detectingcircuit of FIG. 1;

FIG. 8 is a diagram showing another example of the configuration of thedifferentiating circuit provided in the motor rotation rate detectingcircuit of FIG. 1;

FIG. 9 is a diagram showing another example of the configuration of thedifferentiating circuit provided in the motor rotation rate detectingcircuit of FIG. 1;

FIG. 10 is a diagram showing another example of the configuration of thedifferentiating circuit provided in the motor rotation rate detectingcircuit of FIG. 1;

FIG. 11 is a diagram showing another example of the configuration of thedifferentiating circuit provided in the motor rotation rate detectingcircuit of FIG. 1; and

FIG. 12 is a diagram showing an example of the configuration of thedifferentiating circuit provided in a conventional motor rotation ratedetecting circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. FIG. 1 shows an example of a motor rotationrate detecting circuit embodying the invention. The motor rotation ratedetecting circuit shown in FIG. 1 is fed with two-phase sinusoidalsignals A and B 90° out of phase with each other that are output fromhole elements or the like provided in a motor or from an encoder coupledto a motor.

The sinusoidal signal A is fed to an inverting circuit 1, which invertsthe polarity of the sinusoidal signal A and outputs a sinusoidal signalA overscored. The sinusoidal signal B is fed to an inverting circuit 2,which inverts the polarity of the sinusoidal signal B and outputs asinusoidal signal B overscored.

The four sinusoidal signals, namely A, B, A overscored, and Boverscored, are fed to a switching circuit 3, which selects one amongthose four signals and feeds it to a differentiating circuit 5. Three ofthe sinusoidal signals, namely A, B, and B overscored, are fed to alogic selecting circuit 4, which controls the timing of the switchingperformed by the switching circuit.

The differentiating circuit 5 differentiates the signal output from theswitching circuit 3, and feeds the differentiated signal to adifferentiation error eliminating circuit 6. The differentiation erroreliminating circuit 6 eliminates a differentiation error component fromthe differentiated signal output from the differentiating circuit 5, andoutputs a rotation rate detection signal. According to the rotation ratedetection signal output from the differentiation error eliminatingcircuit 6, a selecting circuit 7 turns on and off an analog switch (notshown) provided within the differentiating circuit 5.

FIG. 2 shows an example of the configuration of the inverting circuits 1and 2. The inverting circuit shown in FIG. 2 is composed of resistors 21and 23 and an operational amplifier 22. One end of the resistor 21serves as an input end of the inverting circuit. The other end of theresistor 21 is connected to the inverting input terminal of theoperational amplifier 22. The non-inverting input terminal of theoperational amplifier 22 is grounded. The operational amplifier 22receives negative feedback through the resistor 23. The node at whichthe operational amplifier 22 and the resistor 23 are connected togetherserves as the output end of the inverting circuit.

FIG. 3 shows an example of the configuration of the switching circuit 3and the logic selecting circuit 4. The switching circuit 3 is composedof analog switches 31 to 34. The analog switch 31 receives at one endthereof the sinusoidal signal A, the analog switch 32 receives at oneend thereof the sinusoidal signal B, the analog switch 33 receives atone end thereof the sinusoidal signal A overscored, and the analogswitch 34 receives at one end thereof the sinusoidal signal Boverscored. The analog switches 31 to 34 are at the other end thereofconnected together to serve as the output end of the switching circuit3. The switching circuit 3 outputs at the output end thereof a signalSC.

The logic selecting circuit 4 is composed of operational amplifiers 41and 42, inverter circuits 43 and 44, and NOR circuits 45 to 48. Theoperational amplifier 41 receives at the non-inverting input terminalthereof the sinusoidal signal B, and receives at the inverting inputterminal thereof the sinusoidal signal A. The operational amplifier 41outputs a rectangular signal that is high when the sinusoidal signal Bis greater than the sinusoidal signal A and low when the sinusoidalsignal B is not greater than the sinusoidal signal A. The operationalamplifier 42 receives at the non-inverting input terminal thereof thesinusoidal signal A, and receives at the inverting input terminalthereof the sinusoidal signal B overscored. The operational amplifier 42outputs a rectangular signal that is high when the sinusoidal signal Ais greater than the sinusoidal signal B overscored and low when thesinusoidal signal A is not greater than the sinusoidal signal Boverscored.

The rectangular signal output from the operational amplifier 41 is fedto the inverter circuit 43, which inverts and outputs the signal. Therectangular signal output from the operational amplifier 42 is fed tothe inverter circuit 44, which inverts and outputs the signal.

The NOR circuit 45 receives the output signal of the operationalamplifier 41 and the output signal of the operational amplifier 42, andfeeds a signal S1 obtained by inverting the OR of those two signals tothe control terminal of the analog switch 31 provided in the switchingcircuit 3. The NOR circuit 46 receives the output signal of theoperational amplifier 41 and the output signal of the inverter circuit44, and feeds a signal S2 obtained by inverting the OR of those twosignals to the control terminal of the analog switch 32 provided in theswitching circuit 3. The NOR circuit 47 receives the output signal ofthe inverter circuit 43 and the output signal of the inverter circuit44, and feeds a signal S3 obtained by inverting the OR of those twosignals to the control terminal of the analog switch 33 provided in theswitching circuit 3. The NOR circuit 48 receives the output signal ofthe operational amplifier 42 and the output signal of the invertercircuit 43, and feeds a signal S4 obtained by inverting the OR of thosetwo signals to the control terminal of the analog switch 34 provided inthe switching circuit 3.

Configured as described above, the logic selecting circuit 4 producescontrol signals S1 to S4 as shown in FIG. 4. Accordingly, the signal SCoutput from the switching circuit 3 has a waveform as shown in FIG. 4which is a combination of the waveform of that portion of each of thesinusoidal signals A, B, A overscored, and B overscored which extendsfrom 45° before to 45° after their respective rising zero-cross point.

The differentiating circuit 5 differentiates the signal SC, and outputsa differentiated signal SD that has a waveform as shown in FIG. 4. Thedifferentiation error eliminating circuit 6 eliminates a differentiationerror component from the differentiated signal SD output from thedifferentiating circuit 5, and outputs a rotation rate detection signalSE. The voltage of the differentiated signal SD is proportional to theangular velocity of the sinusoidal signals A and B, and this makes itpossible to detect the motor rotation rate on the basis of the voltageof the rotation rate detection signal SE.

Next, the differentiating circuit 5 and the selecting circuit 7, whichcharacterize the present invention, will be described in more detail.FIG. 5 shows an example of the configuration of the selecting circuit 7.The selecting circuit shown in FIG. 5 is composed of an operationalamplifier 71, a constant voltage source 72, and an inverter circuit 73.The non-inverting input terminal of the operational amplifier 71 isconnected to the output end of the differentiation error eliminatingcircuit 6 (see FIG. 1) so as to receive the rotation rate detectionsignal SE. The inverting input terminal of the operational amplifier 71is connected to the positive side of the constant voltage source 72 soas to receive a constant voltage. The negative side of the constantvoltage source 72 is grounded.

The output terminal of the operational amplifier 71 is connected to thedifferentiating circuit 5 (see FIG. 1) so as to feed the differentiatingcircuit 5 with a control signal CTL1 that is high when the rotation ratedetection signal SE is greater than the constant voltage and low whenthe rotation rate detection signal SE is not greater than the constantvoltage. The output terminal of the operational amplifier 71 is alsoconnected to the input terminal of the inverter circuit 73. The outputterminal of the inverter circuit 73 is connected to the differentiatingcircuit 5 (see FIG. 1) so as to feed the differentiating circuit 5 witha control signal CTL1 overscored, which is the inversion of the controlsignal CTL1. That is, the selecting circuit 7 feeds the differentiatingcircuit 5 with the control signals CTL1 and CTL1 overscored. The controlsignal CTL1 is high when the motor rotation rate is equal to or higherthan a predetermined value and low when the motor rotation rate is lowerthan the predetermined value.

FIG. 6 shows an example of the configuration of the differentiatingcircuit 5. The differentiating circuit shown in FIG. 6 is composed of acapacitor C1, resistors R1 to R4, analog switches SW1 to SW4, and anoperational amplifier OP1. One end of the capacitor C1 serves as theinput end of the differentiating circuit. The other end of the capacitorC1 is connected to one end of the resistor R1 and to one end of theresistor R3. The other end of the resistor R1 is connected through theanalog switch SW1 to the inverting input terminal of the operationalamplifier OP1, and the other end of the resistor R3 is connected throughthe analog switch SW3 to the inverting input terminal of the operationalamplifier OP1.

Also connected to the inverting input terminal of the operationalamplifier OP1 are one end of the resistor R2 and one end of the resistorR4. The other end of the resistor R2 is connected through the analogswitch SW2 to the output terminal of the operational amplifier OP1, andthe other end of the resistor R4 is connected through the analog switchSW4 to the output terminal of the operational amplifier OP1. Thenon-inverting input terminal of the operational amplifier OP1 isgrounded. The node at which the output terminal of the operationalamplifier OP1 and the analog switches SW2 and SW4 are connected togetherserves as the output end of the differentiating circuit.

The analog switches SW1 and SW2 are controlled by the control signalCTL1 (see FIG. 5) output from the selecting circuit 7, and the analogswitches SW3 and SW4 are controlled by the control signal CTL1overscored (see FIG. 5) output from the selecting circuit 7.

Now, the operation of the differentiating circuit configured asdescribed above will be described. First, how it operates when the motorrotation rate is lower than a predetermined value will be described.When the motor rotation rate is lower then a predetermined value, thecontrol signal CTL1 is low, and the control signal CTL1 overscored ishigh. Accordingly, the analog switches SW1 and SW2 are off, and theanalog switches SW3 and SW4 are on. Thus, let the capacitance of thecapacitor C1 be C₁ [F], the resistance of the resistor R3 be R₃ [Ω], andthe resistance of the analog switch SW3 when it is on (hereinafterreferred to as the on-state resistance) be R_(SW3) [Ω]. Then, thedifferentiating operation frequency of the differentiating circuit shownin FIG. 6 is lower than 1/(2×π×C₁×(R₃+R_(SW3))) [Hz]. Moreover, let theresistance of the resistor R4 be R₄ [Ω] and the on-state resistance ofthe analog switch SW4 be R_(SW4) [Ω]. Then, the alternative-current gainof the differentiating circuit shown in FIG. 6 is(R₄+R_(SW4))/(R₃+R_(SW3)).

Next, how the differentiating circuit operates when the motor rotationrate is equal to or higher than a predetermined value will be described.In this case, the control signal CTL1 is high, and the control signalCTL1 overscored is low. Accordingly, the analog switches SW1 and SW2 areon, and the analog switches SW3 and SW4 are off. Thus, let theresistance of the resistor R1 be R₁ [Ω] and the on-state resistance ofthe analog switch SW1 be R_(SW1) [Ω]. Then, the differentiatingoperation frequency of the differentiating circuit shown in FIG. 6 islower than 1/(2×π×C₁×(R₁+R_(SW1))) [Hz]. Moreover, let the resistance ofthe resistor R2 be R₂ [Ω] and the on-state resistance of the analogswitch SW2 be R_(SW2) [Ω]. Then, the alternative-current gain of thedifferentiating circuit shown in FIG. 6 is (R₂+R_(SW2))/(R₁+R_(SW1)).

Here, the resistances R₁ and R₃ of the resistors R1 and R3 are so set asto fulfill R₁<R₃. Thus, when the motor rotation speed is lower than thepredetermined value, the maximum value of the differentiating operationfrequency is small, and, when the motor rotation speed is equal to orhigher than the predetermined value, the maximum value of thedifferentiating operation frequency is great. This prevents thedifferentiating circuit from amplifying high-frequency noise when themotor is rotating at a low rate, but nevertheless permits the motorrotation rate to be detected when the motor is rotating at a high rate.In this way, it is possible to realize a motor rotation rate detestingcircuit that is capable of wide-range detection with satisfactoryresistance to high-frequency noise.

Moreover, the resistances R₁ to R₄ of the resistors R1 to R4 and theon-state resistances R_(SW1) to R_(SW4) of the analog switches SW1 toSW4 are so set as to fulfill(R₄+R_(SW4))/(R₃+R_(SW3))=(R₂+R_(SW2))/(R₁+R_(SW1)). This makes itpossible keep the alternating-current gain constant irrespective of themotor rotation rate. Thus, the gain of the rotation rate detectionsignal SE remains constant irrespective of the motor rotation rate, andthis saves the motor driving circuit, i.e., the circuit that controlsthe motor according to the rotation rate detection signal SE output fromthe motor rotation rate detecting circuit, from performing extraoperation in addition to the operation it conventionally performs.

Incidentally, in a case where the on-state resistances RSW_(SW1) toR_(SW4) of the analog switches SW1 to SW4 are made extremely lowrelative to the resistances R₁ to R₄ of the resistors R1 to R4 as by theuse of analog switches with low on-state resistances, thedifferentiating operation frequency and the alternating-current gain canbe calculated accurately even if the terms in the respective formulaewhich relate to the on-state resistances of the analog switches areomitted.

FIGS. 7 to 11 show other examples of the configuration of thedifferentiating circuit 5. With any of these differentiating circuits,the resistances of the resistors and the on-state resistances of theanalog switches are so set that, when the motor rotation rate is lowerthan a predetermined value, the maximum value of the differentiatingoperation frequency is small and that, when the motor rotation rate isequal to or higher than the predetermined value, the maximum value ofthe differentiating operation frequency is great. Moreover, theresistances of the resistors and the on-state resistances of the analogswitches are so set that the alternating-current gain remains constantirrespective of the motor rotation rate. In a case where the on-stateresistances of the analog switches are made extremely low relative tothe resistances of the resistor as by the use of analog switches withlow on-state resistances, the on-state resistances of the analogswitches may be ignored.

Now, the configuration of the differentiating circuit shown in FIG. 7will be described. Here, such circuit elements as are found also in thedifferentiating circuit shown in FIG. 6 are identified with the samereference numerals and symbols, and their detailed explanations will notbe repeated. In the differentiating circuit shown in FIG. 7, the analogswitches SW3 and SW4 used in the differentiating circuit shown in FIG. 6are omitted, and instead the resistor R3 is connected directly to theinverting input terminal of the operational amplifier OP1 and theresistor R4 is connected directly to the output terminal of theoperational amplifier OP1. Correspondingly, it is advisable to use aselecting circuit in which the inverter circuit 73 used in the selectingcircuit shown in FIG. 5 is omitted. This helps achieve a simpler circuitconfiguration than in FIG. 6.

Next, the configuration of the differentiating circuit shown in FIG. 8will be described. The differentiating circuit shown in FIG. 8 iscomposed of a capacitor C2, resistors R5 and R6, analog switches SW5 andSW6, and an operational amplifier OP2. One end of the capacitor C2serves as the input end of the differentiating circuit. The other end ofthe capacitor C2 is connected to one end of the resistor R5, to one endof the resistor R6, and to the non-inverting input terminal of theoperational amplifier OP2. The other end of the resistor R5 is groundedthrough the analog switch SW5, and the other end of the resistor R6 isgrounded through the analog switch SW6. The analog switch SW5 iscontrolled by the control signal CTL1 (see FIG. 5) output from theselecting circuit 7, and the analog switch SW6 is controlled by thecontrol signal CTL1 overscored (see FIG. 5) output from the selectingcircuit 7. The inverting input terminal and output terminal of theoperational amplifier OP2 are connected together directly, and the nodeat which they are connected together serves as the output end of thedifferentiating circuit.

Next, the configuration of the differentiating circuit shown in FIG. 9will be described. Here, such circuit elements as are found also in thedifferentiating circuit shown in FIG. 8 are identified with the samereference numerals and symbols, and their detailed explanations will notbe repeated. In the differentiating circuit shown in FIG. 9, thedifferentiating circuit shown in FIG. 8 is further provided withresistors R7 and R8. Here, the inverting input terminal of theoperational amplifier OP2 is grounded through the resistor R7. Moreover,the inverting input terminal and output terminal of the operationalamplifier OP2 are connected together not directly but through theresistor R8, and the node at which the output terminal of theoperational amplifier OP2 and the resistor R8 are connected togetherserves as the output end of the differentiating circuit. This makes itpossible to obtain a higher gain and thereby achieve sure detection evenwhen the error component is small.

Next, the configuration of the differentiating circuit shown in FIG. 10will be described. Here, such circuit elements as are found also in thedifferentiating circuit shown in FIG. 8 are identified with the samereference numerals and symbols, and their detailed explanations will notbe repeated. In the differentiating circuit shown in FIG. 10, the analogswitch SW6 used in the differentiating circuit shown in FIG. 8 isomitted, and instead the resistor R6 is grounded directly.Correspondingly, it is advisable to use a selecting circuit in which theinverter circuit 73 used in the selecting circuit shown in FIG. 5 isomitted.

Next, the configuration of the differentiating circuit shown in FIG. 11will be described. Here, such circuit elements as are found also in thedifferentiating circuit shown in FIG. 9 are identified with the samereference numerals and symbols, and their detailed explanations will notbe repeated. In the differentiating circuit shown in FIG. 11, the analogswitch SW6 used in the differentiating circuit shown in FIG. 9 isomitted, and instead the resistor R6 is grounded directly.Correspondingly, it is advisable to use a selecting circuit in which theinverter circuit 73 used in the selecting circuit shown in FIG. 5 isomitted.

The embodiment described above deals with a case in which adifferentiating circuit is used that switches the maximum value of thedifferentiating operation frequency in two steps by switching analogswitches. It is also possible, however, to use a differentiating circuitthat switches the maximum value of the differentiating operationfrequency in three or more steps.

A motor rotation rate detecting circuit embodying the present inventionmay be configured in any other manner than shown in FIG. 1. For example,it may be configured like the motor rotation rate detecting circuitdisclosed in Japanese Patent Application Laid-Open No. S57-76456,provided that its differentiating circuit is so configured as to permitthe maximum value of the differentiating operation frequency to beswitched in a plurality of steps and that a selecting circuit forcontrolling the switches provided in that differentiating circuit isadditionally provided. The voltage of the rotation rate detection signalSE may be detected in a plurality of steps, with the switches providedin the differentiating circuit so configured as to offer the maximumvalue of the differentiating operation frequency in a plurality ofsteps. In the embodiment described above, the control signal CTL1overscored is produced within the selecting circuit. It is alsopossible, however, to produce the control signal CTL1 overscored,whenever it is necessary, within the differentiating circuit byinverting the control signal CTL1.

In the embodiment described above, the signals output from the encodercoupled to the motor are sinusoidal signals. However, the signals outputfrom the encoder coupled to the motor may be signals having alternatingwaveforms of any other type so long as, by differentiating them, it ispossible to extract a component that indicates the motor rotation rate.

In any of the differentiating circuits shown in FIGS. 8 to 11, theanalog switches SW5 and SW6 may be replaced with switching circuitsusing transistors. For example, it is possible to provide, instead ofthe analog switch SW5, an n-channel MOSFET that has its drain connectedto the resistor R5, that has its source connected to ground, and thatreceives at its gate the control signal CTL1, and provide, instead ofthe analog switch SW6, an n-channel MOSFET that has its drain connectedto the resistor R6, that has its source connected to ground, and thatreceives at its gate the control signal CTL1 overscored.

A motor driving device embodying the present invention is provided witha motor rotation rate detecting circuit as described above and a motor.A motor driving device embodying the present invention includes a motorand a motor rotation rate detecting circuit that produces a rotationrate detection signal by differentiating a plurality of alternatingsignals having different phases that are output according to therotation position of the motor. Here, the frequency response of thedifferentiating operation of the differentiating circuit provided in themotor rotation rate detecting circuit is switched according to thevoltage of the rotation rate detection signal. As a result, it ispossible to make the maximum value of the differentiating operationfrequency small when the motor is rotating at a low rate and therebyprevent the differentiating circuit from amplifying high-frequency noisewhen the motor is rotating at a low rate. On the other hand, it ispossible to make the maximum value of the differentiating operationfrequency great when the motor is rotating at a high rate and therebyachieve sure detection of the motor rotation rate even when the motor isrotating at a high rate. In this way, it is possible to realize a motordriving device incorporating a motor rotation rate detecting circuitthat is capable of wide-range detection with satisfactory resistance tohigh-frequency noise over the entire range.

In an optical disk apparatus, as the disk rotation rate becomesincreasingly high, the range of the frequency of the output signal ofthe encoder coupled to the spindle motor becomes increasingly wide.Thus, a motor driving device embodying the present invention is suitableas a motor driving device for driving a spindle motor in an optical diskapparatus.

Motor driving devices embodying the present invention find applicationnot only in optical disk apparatuses but in electric appliances of anykind provided with a motor wherein the range of the rotation rate of themotor is wide and the rotation rate of the motor needs to be controlled.

1. A motor driving device comprising: a motor; and a motor rotation ratedetecting circuit, wherein said motor rotation rate detecting circuitincludes a differentiating circuit that produces a rotation ratedetection signal by differentiating a plurality of alternating signalshaving different phases that are output according to a rotation positionof said motor, wherein a frequency response of differentiating operationof said differentiating circuit is switchable according to a voltage ofsaid rotation rate detection signal, wherein said motor rotation ratedetecting circuit further includes a selecting circuit, wherein saidselecting circuit switches the frequency response of the differentiatingoperation of said differentiating circuit according to the voltage ofsaid rotation rate detection signal, wherein said differentiatingcircuit includes a capacitor, at least two resistors, at least oneMOSFET, and an operational amplifier, wherein said selecting circuitswitches the frequency response of the differentiating operation of saiddifferentiating circuit by turning on and off said at least one MOSFETaccording to the voltage of said rotation rate detection signal, andwherein said differentiating circuit has a constant alternating-currentgain irrespective of the frequency response of the differentiatingoperation thereof.